This invention relates to hydraulic circuit systems for construction machines, and more particularly to a hydraulic circuit system for a construction machine such as a hydraulic excavator which is equipped with a plurality of working elements.
Generally, a hydraulic excavator is equipped with a plurality of working elements including a swing, left and right travelling tracks, a boom, an arm and a bucket. These working elements are driven by hydraulic actuators such as a swing motor, a pair of travel motors, a boom cylinder, an arm cylinder and a bucket cylinder which are incorporated in a hydraulic circuit system.
Typically, the hydraulic circuit system has been constructed such that the hydraulic actuators are classified into two groups and each group is provided with a separate hydraulic pump to constitute a first hydraulic circuit and a second hydraulic circuit, and the actuators of each hydraulic circuit are connected in parallel with one another through respective directional control valves. The hydraulic circuit system of this construction offers the advantages that the construction of the hydraulic circuits is simple and that a plurality of actuators can be simultaneously driven. However, disadvantages are associated with this system. More particularly, when combined operations are performed for simultaneously driving a plurality of actuators, movements of the actuators connected in parallel with each other might be influenced by the working pressures of the respective actuators and actuators of higher working pressures might have their working speeds reduced or might be rendered inoperative because of the hydraulic fluid flowing into actuators of lower working pressures.
To obviate such problems, a proposal has been made in U.S. Pat. No. 4,112,821 to connect in tandem a plurality of directional control valves in each hydraulic circuit to ensure that each actuator operates independently of other actuators. More specifically, the hydraulic circuit system disclosed in this U.S. patent comprises first and second hydraulic circuit means, and the first hydraulic circuit means has a swing directional control valve, a left travel directional control valve and an arm directional control valve which are connected in tandem with one another in the indicated order with respect to a first hydraulic pump to constitute a first valve group, while the second hydraulic circuit means has a right travel directional control valve, a bucket directional control valve and a boom directional control valve which are connected in tandem with one another with respect to a second hydraulic pump in the indicated order to constitute a second valve group. Center bypass lines of the first and second valve groups are connected to a reservoir and each have an on-off valve mounted therein. A plurality of bypass circuits are formed between predetermined positions in the first and second hydraulic circuit means in order to avoid the defects which the system might suffer on account of the tandem connections for ensuring independent operations of the swing motor, left travel motor, arm cylinder, right travel motor, bucket cylinder and boom cylinder.
Thus, in the hydraulic circuit system of the aforesaid construction, the swing can operate completely independently of the boom and bucket and can operate independently to a certain degree with respect to the arm due to the provision of a flow restrictor in the associated bypass circuit. Also, by virtue of the action of other bypass circuits, it is possible to perform combined operations of two actuators in the same hydraulic circuit means such as operation of the arm cylinder while operating the swing motor, and to perform combined operations of three actuators, such as operation of the swing motor while operating the left and right travel motors.
Thus, the hydraulic circuit system disclosed in the above noted U.S. patent referred has succeeded to a certain extent in accomplishing the object of performing combined operations of a plurality of actuators while ensuring that the actuators operate independently of one another. However, this system is faced with the problem that, since the directional control valves for the actuators are essentially connected in tandem with one another, limits are placed on the range of combined operations of the actuators and the operability of the system would not be so high. For example, since the boom directional control valve and the bucket directional control valve are connected in tandem with the right travel directional control valve at a location downstream thereof, a boom raising operation or a bucket raising operation can not be performed while travelling is performed. Also, although the arm operation during travelling can be performed by the action of the bypass circuit, it would be impossible to perform an arm operation satisfactorily when the working pressure of the right travel motor is low since hydraulic fluid from the first pump would flow into the left travel motor. It would be impossible to steer the vehicle by operating the left travel motor during swing operation, since the on-off valve mounted in the center bypass line of the second valve group is held in an open position to allow hydraulic fluid from the second pump to directly flow into the reservoir and keep hydraulic fluid from being supplied to the left travel motor.